In animals, circulating levels of the hormone melatonin (also known chemically as N-acetyl-5-methoxytryptamine) vary in a daily cycle, thereby allowing the entrainment of the circadian rhythms of several biological functions. Melatonin is produced in humans by the pineal gland, a small endocrine gland located in the center of the brain. The melatonin signal forms part of the system that regulates the sleep-wake cycle by chemically causing drowsiness and lowering the body temperature. Melatonin is commonly released in darkness (roughly 4-5 hours before sleep), and its production is suppressed by exposure to light. The light-dependent character of melatonin release and suppression aids in falling asleep and waking up. Depending on the amount, melatonin can reduce core body temperature and induce sleepiness. Conversely, nighttime light exposure can increase body temperature, and enhance alertness and performance.
It is principally blue light (e.g., including blue light at a peak wavelength value between 460 to 480 nm, with some activity from about 360 nm to about 600 nm), that suppresses melatonin and synchronizes the circadian clock, proportional to the light intensity and length of exposure. As shown in FIG. 1, the action spectrum for melatonin suppression (with six individual data points represented as black squares) shows short-wavelength sensitivity that is very different from the known spectral sensitivity of the scotopic response curve (represented with a solid line) and photopic response curve (represented with a dashed line)—being shifted approximately 50 nm and 100 nm to the left of the scotopic and photopic response, respectively. (FIG. 1 was originally presented in Thapan, Kavita, et al., “An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans,” J. Physiol. (2001), 535.1, pp. 261-267.)
Circadian rhythm disorders may be associated with change in nocturnal activity (e.g., nighttime shift workers), change in latitude (e.g., jet lag), and/or seasonal change in light duration (e.g., seasonal affective disorder, with symptoms including depression). The World Health Organization in 2007 named late night shift work as a probable cancer-causing agent. Melatonin is an anti-oxidant and suppressant of tumor development; accordingly, interference with melatonin levels may increase likelihood of developing cancer. It would be desirable to ameliorate or reduce symptoms of circadian rhythm disorders and other health conditions that may be associated with reduced melatonin levels.
With proliferation of tablet computers, electronic readers, and other backlit electronic devices, consumers are increasingly utilizing backlit devices at nighttime hours, with the attendant potential for melatonin suppression. Some consumers have reported that reading textual content using backlight electronic devices reduces sensation of drowsiness and/or interferes with falling asleep normally, in a manner not experienced by reading conventional books. Although certain backlight devices (e.g., computer monitors) permit users to control backlight color temperature, certain backlight color temperatures are not aesthetically pleasing to certain users.
Solid state light sources such as organic or inorganic light emitting diodes (LEDs) or lasers may be used to provide colored (e.g., non-white) or white light (e.g., perceived as being white or near-white). White solid state emitters are increasingly being used potential replacements for white incandescent or fluorescent lamps for reasons including substantially increased efficiency and longevity. Solid state light sources provide potential for very high efficiency relative to conventional incandescent or fluorescent sources, but have presented challenges in simultaneously achieving good efficacy, good color reproduction, color variation among different emitters, and color stability (e.g., with respect to variations in operating temperature).
Color reproduction is commonly measured using Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). In calculating CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference illuminant and the test source, and a difference in color appearance for each sample between the test and reference illumination is computed. CRI therefore provides a relative measure of the shift in surface color and brightness of an object when lit by a particular lamp. The general color rendering index CRI Ra is a modified average utilizing the first eight indices, all of which have low to moderate chromatic saturation. The CRI Ra equals 100 (a perfect score) if the color coordinates and relative brightness of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80) for general illumination use where the colors of objects are not important. For some general interior illumination, a CRI Ra>80 is acceptable. CRI Ra>85, and more preferably, CRI Ra>90, provides greater color quality.
Aspects relating to the present inventive subject matter may be better understood with reference to the 1931 CIE (Commission International de l'Eclairage) Chromaticity Diagram and/or the 1976 CIE Chromaticity Diagram, both of which are well-known and readily available to those of ordinary skill in the art. The CIE Chromaticity Diagrams map out the human color perception in terms of two CIE parameters x and y (in the case of the 1931 diagram) or u′ and v′ (in the case of the 1976 diagram). The 1931 CIE Chromaticity Diagram is reproduced at FIG. 2, and the 1976 CIE Chromaticity Diagram (also known as (u′v′) chromaticity diagram) is reproduced at FIG. 3. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors. The 1976 CIE Chromaticity Diagram is similar to the 1931 Diagram, except that the 1976 Diagram has been modified such that similar distances on the Diagram represent similar perceived differences in color. Since similar distances on the 1976 Diagram represent similar perceived differences in color, deviation from a point on the 1976 Diagram can be expressed in terms of the coordinates, u′ and v′, e.g., distance from the point=(Δu′2+Δv′2)′1/2, and the hues defined by a locus of points that are each a common distance from a specified hue consist of hues that would each be perceived as differing from the specified hue to a common extent.
The chromaticity coordinates (i.e., color points) that lie along the blackbody locus (“BBL”) obey Planck's equation: E(λ)=A λ−5/(eB/T−1), where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody, and A and B are constants. Color coordinates that lie on or near the BBL yield pleasing white light to a human observer. The 1931 CIE Diagram (FIG. 2) includes temperature listings along the blackbody locus (embodying a curved line emanating from the right corner). These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants which produce light that is on or near the BBL can thus be described in terms of their color temperature.
The term “white light” or “whiteness” does not clearly cover the full range of colors along the BBL since it is apparent that a candle flame and other incandescent sources appear yellowish, i.e., not completely white. Accordingly, the color of illumination may be better defined in terms of correlated color temperature (CCT) and in terms of its proximity to the BBL. The pleasantness and quality of white illumination decreases rapidly if the chromaticity point of the illumination source deviates from the BBL by a distance of greater than 0.01 in the x, y chromaticity system. This corresponds to the distance of about four MacAdam ellipses, a standard employed by the lighting industry. A lighting device emitting light having color coordinates that are within four MacAdam step ellipses of the BBL and that has a CRI Ra>80 is generally acceptable as a white light for illumination purposes. A lighting device emitting light having color coordinates within seven or eight MacAdam ellipses of the BBL and that has a CRI Ra>70 is used as the minimum standards for many other white lighting devices including compact fluorescent and solid state lighting devices. General illumination generally has a color temperature between 2,000 K and 10,000 K, with the majority of lighting devices for general illumination being between 2,700 K and 6,500 K.
The art continues to seek improved lighting devices that address one or more limitations inherent to conventional devices.